Bottom Line:
A number of genes have been identified affecting mucilage secretory cell differentiation, including MUCILAGE-MODIFIED4 (MUM4). mum4 mutants produce a reduced amount of mucilage and cloning of MUM4 revealed that it encodes a UDP-L-rhamnose synthase that is developmentally up-regulated to provide rhamnose for mucilage pectin synthesis.Eight mum enhancers (men) have been identified, two of which result from defects in known mucilage secretory cell genes (MUM2 and MYB61).Conversely, mutations in MEN2 and MEN6 appear to affect mucilage release rather than quantity.

ABSTRACTPollination triggers not only embryo development but also the differentiation of the ovule integuments to form a specialized seed coat. The mucilage secretory cells of the Arabidopsis thaliana seed coat undergo a complex differentiation process in which cell growth is followed by the synthesis and secretion of pectinaceous mucilage. A number of genes have been identified affecting mucilage secretory cell differentiation, including MUCILAGE-MODIFIED4 (MUM4). mum4 mutants produce a reduced amount of mucilage and cloning of MUM4 revealed that it encodes a UDP-L-rhamnose synthase that is developmentally up-regulated to provide rhamnose for mucilage pectin synthesis. To identify additional genes acting in mucilage synthesis and secretion, a screen for enhancers of the mum4 phenotype was performed. Eight mum enhancers (men) have been identified, two of which result from defects in known mucilage secretory cell genes (MUM2 and MYB61). Our results show that, in a mum4 background, mutations in MEN1, MEN4, and MEN5 lead to further reductions in mucilage compared to mum4 single mutants, suggesting that they are involved in mucilage synthesis or secretion. Conversely, mutations in MEN2 and MEN6 appear to affect mucilage release rather than quantity. With the exception of men4, whose single mutant exhibits reduced mucilage, none of these genes have a single mutant phenotype, suggesting that they would not have been identified outside the compromised mum4 background.

fig6: Time-course of germination of mum4 enhancers and controls. Genotypes are organized in the same order as Fig. 4 for comparison. Seeds were stratified for 3 d at 4 °C, followed by germination at 22 °C under 16/8 h light/dark. 40–80 seed of each genotype were sowed on filter paper with water, error bars=SE. Similar results were obtained in a separate experiment using seed from an independent set of plants.

Mentions:
Altered seed germination responses have been correlated with changes in seed coat structure, including mucilage quantity and release (Léon-Kloosterziel et al., 1994; Debeaujon et al., 2000; Penfield et al., 2001; Rautengarten et al., 2008). To determine the effect of reduced mucilage levels in the mum4 single mutant, as well as in the men lines, a time-course of germination was performed (Fig. 6). mum4-1 germination lagged significantly behind that of wild-type seeds at 3 d (23% for mum4-1 versus 67% for Col-2), but reached approximately wild-type levels by 4 d (Fig. 6). Similar, or even more severe delays, at 3 d were detected for the set of men mum4-1 lines plus myb61-6 mum4-1 and mum2-1 mum4-1 double mutants, all of which continued to stay significantly below wild-type germination levels at day four, with the exception of men4 mum4 (Fig. 6). All lines reached 95–100% germination within 9 d (data not shown). Together, these results suggest not only that the reduction of mucilage in mum4-1 has an effect on the speed of germination, but also that this delay may be enhanced by further defects in both mucilage release and quantity. Conversely, men4-1, which has approximately three times more mucilage than mum4-1 (Fig. 4), shows no delay in germination. Interestingly, both men4-1 mum4-1 and men5-1 mum4-1 seeds demonstrate precocious germination relative to mum4-1 at 2 d. This is reflected in men4-1, which germinates faster than the wild type at 2 d. This ‘early germination’ may explain the similar if not faster germination exhibited by men4-1 versus wild type, and men4-1 mum4-1 and men5-1 mum4-1 versus mum4-1 exhibited at 3 d. It is possible that, in these lines, there is a germination phenotype beyond that resulting from reduced mucilage levels.

fig6: Time-course of germination of mum4 enhancers and controls. Genotypes are organized in the same order as Fig. 4 for comparison. Seeds were stratified for 3 d at 4 °C, followed by germination at 22 °C under 16/8 h light/dark. 40–80 seed of each genotype were sowed on filter paper with water, error bars=SE. Similar results were obtained in a separate experiment using seed from an independent set of plants.

Mentions:
Altered seed germination responses have been correlated with changes in seed coat structure, including mucilage quantity and release (Léon-Kloosterziel et al., 1994; Debeaujon et al., 2000; Penfield et al., 2001; Rautengarten et al., 2008). To determine the effect of reduced mucilage levels in the mum4 single mutant, as well as in the men lines, a time-course of germination was performed (Fig. 6). mum4-1 germination lagged significantly behind that of wild-type seeds at 3 d (23% for mum4-1 versus 67% for Col-2), but reached approximately wild-type levels by 4 d (Fig. 6). Similar, or even more severe delays, at 3 d were detected for the set of men mum4-1 lines plus myb61-6 mum4-1 and mum2-1 mum4-1 double mutants, all of which continued to stay significantly below wild-type germination levels at day four, with the exception of men4 mum4 (Fig. 6). All lines reached 95–100% germination within 9 d (data not shown). Together, these results suggest not only that the reduction of mucilage in mum4-1 has an effect on the speed of germination, but also that this delay may be enhanced by further defects in both mucilage release and quantity. Conversely, men4-1, which has approximately three times more mucilage than mum4-1 (Fig. 4), shows no delay in germination. Interestingly, both men4-1 mum4-1 and men5-1 mum4-1 seeds demonstrate precocious germination relative to mum4-1 at 2 d. This is reflected in men4-1, which germinates faster than the wild type at 2 d. This ‘early germination’ may explain the similar if not faster germination exhibited by men4-1 versus wild type, and men4-1 mum4-1 and men5-1 mum4-1 versus mum4-1 exhibited at 3 d. It is possible that, in these lines, there is a germination phenotype beyond that resulting from reduced mucilage levels.

Bottom Line:
A number of genes have been identified affecting mucilage secretory cell differentiation, including MUCILAGE-MODIFIED4 (MUM4). mum4 mutants produce a reduced amount of mucilage and cloning of MUM4 revealed that it encodes a UDP-L-rhamnose synthase that is developmentally up-regulated to provide rhamnose for mucilage pectin synthesis.Eight mum enhancers (men) have been identified, two of which result from defects in known mucilage secretory cell genes (MUM2 and MYB61).Conversely, mutations in MEN2 and MEN6 appear to affect mucilage release rather than quantity.

ABSTRACTPollination triggers not only embryo development but also the differentiation of the ovule integuments to form a specialized seed coat. The mucilage secretory cells of the Arabidopsis thaliana seed coat undergo a complex differentiation process in which cell growth is followed by the synthesis and secretion of pectinaceous mucilage. A number of genes have been identified affecting mucilage secretory cell differentiation, including MUCILAGE-MODIFIED4 (MUM4). mum4 mutants produce a reduced amount of mucilage and cloning of MUM4 revealed that it encodes a UDP-L-rhamnose synthase that is developmentally up-regulated to provide rhamnose for mucilage pectin synthesis. To identify additional genes acting in mucilage synthesis and secretion, a screen for enhancers of the mum4 phenotype was performed. Eight mum enhancers (men) have been identified, two of which result from defects in known mucilage secretory cell genes (MUM2 and MYB61). Our results show that, in a mum4 background, mutations in MEN1, MEN4, and MEN5 lead to further reductions in mucilage compared to mum4 single mutants, suggesting that they are involved in mucilage synthesis or secretion. Conversely, mutations in MEN2 and MEN6 appear to affect mucilage release rather than quantity. With the exception of men4, whose single mutant exhibits reduced mucilage, none of these genes have a single mutant phenotype, suggesting that they would not have been identified outside the compromised mum4 background.